Directional couplers are circuit elements of high-frequency (HF) and aerial engineering and are used for asymmetrically dividing up power, e.g. at a magnitude of −12 dB, in a desired frequency range. Directional couplers always have a short wire segment, whose characteristic impedance corresponds to that of the utilized electric line. In this manner, a specific voltage is only decoupled from the forward wave or return wave.
A directional coupler relevant in this case is found in an article published on Dec. 5, 2003, having the title “HF-Passive Komponenten” [“Passive HF Components”], by Professor D. U. Gysel, ZHW, Department of Technology, Information Technology and Natural Sciences, Electrical Engineering and Signal Processing, High-Frequency Engineering, Zurich, and is schematically represented in FIG. 1, which is described below in detail.
As can be gathered from FIG. 1, directional couplers are designed to have four ports and have two receiving ports (input ports) and two transmitting ports (output ports). The two receiving ports must be decoupled from one another as much as possible. The stripline directional couplers in question here are produced, using conventional printed circuit technique. In this context, substrates having relatively low dielectric constants, as well as coupling gaps between the two conductors, having a very small gap width in the range of approximately 100 μm, are used in order to attain the desired, high coupling values of over 15 dB, for example, 12 dB. Thus, for a 12 dB coupler at 2.5 GHz on a conductor substrate having a thickness of 300 μm and a relative dielectric constant of 4.4, one only obtains a coupling gap width of approximately 80 μm, necessary for the above-mentioned coupling strength. Using today's printed circuit technology, such a small conductor gap can only be produced at a very high cost and degree of difficulty, and a simultaneously high reject rate.
Therefore, there is considerable need for being able to suitably produce directional couplers of the type in question here, using conventional printed circuit technology, which have minimal conductor widths and lateral conductor gaps in the range of 150 μm, as well as etching tolerances of up to +/−20 μm.